Squid formed on a sapphire substrate and method for manufacturing the same

ABSTRACT

The present invention relates to a SQUID made of an oxide superconducting thin film is formed on a sapphire substrate. CeO 2  film, RBa 2 Cu 3 O 7−x  film (“R” indicates a rare earth element chosen among a group formed Yb, Er, Ho, Y, Dy, Gd, Eu, Sm and Nd) and SrTiO 3  film are deposited the substrate top of sapphire successively. Furthermore, an oxide superconducting thin film to be a SQUID is deposited on the SrTiO 3  film.

DETAILED DESCRIPTION OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to of SQUID. The present inventionis related to a configuration of SQUID and a method for manufacturingthe same. More specifically, tie present invention is related to a SQUIDformed of an oxide superconducting thin film on a sapphire substrate.

[0003] 2. Description of related art

[0004] Generally, a SQUID comprises a circular current road forsuperconducting current, including one or two Josefson conjugation(s).The SQUID related to the present invention is a SQUID formed of an oxidesuperconducting thin film in particular that has the composition“RBa₂Cu₃O_(7−x) (“R” indicates a rare earth element chosen among a groupformed Yb, Er, Ho, Y, Dy, Gd, Eu, Sm and Nd)”. The oxide superconductingthin film of this kind has high critical temperature and is to beeffective by cooling by liquid nitrogen.

[0005] However, a specified crystal structure is required so that theoxide thin film obtains superconducting state. Accordingly, in mostcase, an oxide superconducting thin film is formed on a MgO singlecrystal substrate or SrTiO₃ single crystal substrate. These substratematerials have well matching of cell to the oxide superconducting thinfilm and preferable arrangement of crystal is provided.

[0006] However, MgO single crystal substrate and SrTiO₃ single crystalsubstrate are very expensive. And, these substrates having large areaare hard to be obtained. As the result, SQUID formed of an oxidesuperconducting thin film tends to be expensive.

[0007] On the other hand, Si single crystal substrate or sapphiresubstrate is obtained easily and cheep. However, an oxidesuperconducting thin film is hard to be formed on them.

SUMMARY OF THE INVENTION

[0008] The problems mentioned above will be solved by the presentinvention. In manufacture method of the present invention, sapphiresubstrate is used as a substrate material and oxide superconducting thinfilm of high quality is formed at the same time.

[0009] Characteristic of the present invention is not simpledisplacement of substrate material. Namely, when a SrTiO₃ film is formeddirectly on a sapphire substrate, SrTiO₃ film (100) is never formed.However, the method according to the present invention contains peculiarcharacteristic 3 phases production process.

[0010] As the first process, a CeO₂ (100) film is formed on the sapphiresubstrate. As the second process forms, a RBa₂Cu₃O_(7−x) (001) film (“R”indicates a rare earth element chosen among a group formed Yb, Er, Ho,Y, Dy, Gd, Eu, Sm and Nd) is formed on the CeO₂ (100) film. As the thirdprocess, SrTiO₃ (100) film is formed, on the RBa₂Cu₃O_(7−x) (001) film.At last, the oxide superconducting thin film to be a SQUID is formed onthis SrTiO₃ (100) film.

[0011] Such processes bring the following effect.

[0012] Each film sticks well mutually. The oxide superconducting thinfilm can be formed on SrTiO₃ film that is oriented (100). Accordingly,high quality oxide superconducting thin film is provided on a sapphiresubstrate. The quality of the oxide superconducting thin film is equalto the one of an oxide superconducting thin film on MgO substrate orSrTiO₃ substrate Acquisition of a sapphire substrate is easy, differentfrom MgO substrate or SrTiO₃ plate

[0013] Furthermore, an advantage is not simply confined to reduction ofmaterial cost. A sapphire substrate having large area is easy toprovided. Accordingly, several SQUIDs can be formed on one substrate andproduction of SQUID becomes in large quantities

[0014] In other words, high performance and inexpensive SQUID is to besupplied by the present invention.

[0015] The above and other objects, features and advantages of thepresent invention will be apparent from the following description ofpreferred embodiments of the invention with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 shows diagrammatic sectional views for illustrating eachprocess of the method of the present invention.

[0017]FIG. 2 shows a birdseye view of one of embodiments of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Embodiment 1

[0019] At first, as shown in FIG. 1A, CeO₂ film 2 is deposited the wholesurface of the sapphire substrate 1 having a flat R principal surface.As shown in FIG. 1B, a RBa₂Cu₃O_(7−x) (“R” indicates a rare earthelement chosen among a group formed Yb, Er, Ho, Y, Dy, Gd, Eu, Sm andNd) film 3 and a SrTiO₃ film 4 deposited on the CeO₂ film 2successively. Each process of these series processes can be done bypulsed laser deposition for example.

[0020] Successively, the surface of the SrTiO₃ film is processed to forma physical step by the following process.

[0021] At first, as shown in FIG. 1C, resist 5 is deposited on the wholesurface of the SrTiO₃ film 4. Successively, as shown in FIG. 1D, after apart of this resist has exposed, unnecessary domain of the resist isremoved. As shown in FIG. 1E, the resist 5 left is used as a mask whilea part of the SrTiO₃ film 4 is removed by Ar ion-milling. Finally, asshown in FIG. 1F, the resist layer 5 left on the SrTiO₃ film 4 isremoved with appropriate solvent. A kind of solvent is selectedaccording to a kind of resist, for example, a solvent can be an acetone.

[0022] Then, after series of the process, a physical step is formed onthe surface of the SrTiO₃ film 4. This SrTiO₃ film 4 becomes a substrateof an oxide superconducting thin film.

[0023] As shown in FIG. 1G, an oxide superconducting thin film 6 isdeposited on the SrTiO₃ film 4. This oxide superconducting thin film 6has a composition RBa₂Cu₃O_(7−x). (“R” indicates a rare earth elementchosen among a group formed Yb, Er, Ho, Y, Dy, Gd, Eu, Sm and Nd)

[0024] Furthermore, as shown in FIG. 1H, a pair of electrode 7 a, 7 b isformed on the oxide superconducting thin film 6. The electrode 7 a, 7 bis formed by using a metal mask for example and formed of Au membraneand Ag membrane laminating each other for example.

[0025] Finally, the oxide superconducting thin film 6 is processedprecision at the point as same as the step. The oxide superconductingthin film processed forms a Josefson junction. This minute processingcan be enforced by standard photolithography technology.

[0026] Thus, the process of manufacturing the SQUID has completed inthis way. Orientation of each film is shown in Table 1. TABLE 1 CeO₂RBa₂Cu₃O_(7-x) SrTiO₃ (100) (001) (100)

[0027] We made a SQUID actually by the method mentioned above.Deposition of each film was done by laser beam vapor deposition. Commoncondition is shown in Table 2 and individual condition is shown in Table3. TABLE 2 Substrate and distance of target 100 mm Energy density oflaser beam   2.5 J/cm² Exposure area 2 mm × 4 mm

[0028] TABLE 3 Composition of film CeO₂ RBa₂Cu₃O_(7-x) SrTiO₃temperature of substrate (° C.) 680  700 700 oxygen pressure (mTorr) 10400 100 film thickness (nm) 20 100 300

[0029] It was identified by a test that the condition mentioned above iseffective when any chemical element is selected as an element “R”. (“R”indicates a rare earth element chosen among a group formed Yb, Er, Ho,Y, Dy, Gd, Eu, Sm and Nd)

[0030] The height of the step formed on the SrTiO₃ film was 160 nm.Thickness of the oxide superconducting thin film was 220 nm. The widthof the superconducting thin film at the Josefson junction was 5 μm. Theconfiguration of SQUID was 5 mm square. The inductance of SQUID wasturned into 40 pH.

[0031] Current/voltage characteristic and magnetic field/voltagecharacteristic of this SQUID provided were measured by quadrupolemethod. As a result, 21 c was 100 μA, and current potentialcharacteristic of RSJ type was observed. These results mean that theconjugation is good condition. Furthermore, width of voltage modulationby having hanged magnetic field was more than 10 μV.

[0032] Then, we understood that the SQUID formed on the sapphiresubstrate worked as a magnetism sensor and confirmed good characteristicof the SQUID from these resultant.

[0033] Embodiment 2

[0034] Sapphire substrate having large sapphire is ordinary supplied.Accordingly, in case that sapphire substrate is used as a substrate forSQUID, as shown in FIG. 2, several SQUID can be put on one substrate 1.In this case, manufacture time for one SQUID is shortened. Dimension ofeach element drawn in FIG. 2 is exaggerated so that characteristic ofeach element is easy to be understood. At the same time, in FIG. 2, thereference number given for each element is the same as one correspondingin FIG. 1.

1. A method for manufacturing a SQUID formed of oxide superconductingthin film comprising following each processes; (1) CeO₂ film,RBa₂Cu₃O_(7−x) film and SrTiO₃ film are deposited on a sapphiresubstrate in sequence, (“R” indicates a rare earth element chosen amonga group formed Yb, Er, Ho, Y, Dy, Gd, Eu, Sm and Nd) (2) A physical stepis formed on said SrTiO₃ film, (3) A second RBa₂Cu₃O_(7−x) film isformed on said SrTiO₃ film, (“R” indicates as same as the above) and (4)An oxide superconducting thin film is patronized to be a SQUID.
 2. Amethod mentioned in claim 1 , said CeO₂ film is a CeO₂ film oriented(100), said RBa₂Cu₃O_(7−x) film is a RBa₂Cu₃O_(7−x) film oriented (001)(“R” indicates as same as the above) and said SrTiO₃ film is a SrTiO₃film oriented (100).
 3. A method mentioned in claim 2 , said CeO₂ filmis deposited by a laser beam vapor deposition method in an atmosphereless than 50 m Torr.
 4. A method mentioned in claim 3 , said SrTiO₃ filmis deposited by a laser beam vapor deposition method in an atmosphereless than 50 m Torr.
 5. A method mentioned in claim 1 , several SQUIDsare formed on one sapphire substrate simultaneously.
 6. A SQUID formedof oxide superconducting thin film comprising; a sapphire substrate, aCeO₂ film, a RBa₂Cu₃O_(7−x) film and a SrTiO₃ film repeated on saidsapphire substrate and a second RBa₂Cu₃O_(7−x) film (“R” indicates arare earth element chosen among a group formed Yb, Er, Ho, Y, Dy, Gd,Eu, Sm and Nd) that is an oxide superconducting thin film and formed tobe a SQUID.
 7. A SQUID mentioned in claim 6 , said CeO₂ film is a CeO₂film oriented (100), said RBa₂Cu₃O_(7−x) film is a RBa₂Cu₃O_(7−x) filmoriented (001) (“R” indicates as same as the above) and said SrTiO₃ filmis a SrTiO₃ film oriented (100).